Co-pending co-owned application Ser. No. 338,685, filed Mar. 7, 1973, discloses a closure for a medical liquid container having a cap which is separately formed with a thin integral brim overlying and hermetically sealed to the neck flange of the container. An inner boundary of the annular fusion joint provides a line of structural weakness which is ruptured when a breakaway ring, or jacking ring, is threaded downwardly over the cap and into forceful engagement with the brim of the cap. The intermeshing threads of the cap and breakaway ring are left-handed; therefore, a user simply rotates the breakaway ring (which has the appearance of an external cap) in a conventional counterclockwise direction when the container is to be opened. Because of the left-handed nature of the threads, the breakaway ring is actually forced downwardly towards the brim and neck flange of the container and it is the inner cap which is forced upwardly. Such upward force results in a rupture of the brim along its annular line of weakness and the combined cap and brim are then removed as a unit from the container.
While such a construction is highly effective in maintaining a bacteria-tight seal and also insures that the mouth of the dispensing outlet will not become contaminated by the brim of the cap as the cap is removed, the twisting force necessary to rupture the brim along its line of weakness has been found to be undesirably high. To reduce the twisting force which a user must apply to cause such rupture, the brim might be substantially reduced in thickness and might even be scored; however, the advantage of possible reduction in operating torque which such modifications might provide would be more than offset by the greater difficulties of manufacture and the increased possibilities of bacterial invasion and contamination should the brim inadvertently separate during normal handling, processing, and storage.
The following patents are further illustrative of the efforts of others in this field: U.S. Pat. Nos. 3,730,372, 3,597,793, 2,317,420, 3,517,847, 3,767,076, 2,140,732, 3,394,831, 3,393,818, 2,851,201, 2,040,789, 3,443,711, 3,025,988, 3,640,417, 3,108,708, 3,804,282, 3,058,631, and 3,784,282.